While the previous patch is required for proper conformance,
the vast majority of target insns are MVC and XC for implementing
memmove and memset respectively.  The next most common are CLC,
TR, and SVC.

Implementing these (and a few others for which we already have
an implementation) directly is faster than going through full
translation to a TB.
Reviewed-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NRichard Henderson <rth@twiddle.net>